This invention relates generally to nuclear reactors and more particularly, to apparatus for securing jet pump sensing lines within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
In a BWR, hollow tubular jet pumps positioned within the shroud annulus provide the required reactor core water flow. Water, generally under high pressure, flows through the jet pump diffuser from the smaller diameter end to the larger diameter end. The water exits the diffuser at the larger diameter end and is discharged through the adapter assembly into the lower core plenum for circulation through the reactor. The pressure drop in the water flowing through the diffuser is continually monitored. Relatively small pressure sensing lines are mounted to the exterior of each diffuser, and are in fluid communication with the water in the jet pump through the wall of the diffuser so that the pressure may be sensed.
The flow through and around the jet pump contains pressure fluctuations from various sources in the reactor. The pressure fluctuations can have frequencies close to one or more natural vibration modes of the sensing line piping depending on the spacing and stiffness of welded supports attaching the sensing line piping to the jet pump. Particularly, in some known jet pumps, there is a relatively long unsupported length of the sensing line routing from a radiused bend below a lower support block to an anchor point at the next adjacent jet pump. When an excitation frequency is close to matching the sensing line natural frequency at some particular location, vibration of the sensing line exerts loads on its support attachments which can cause cyclic fatigue cracking and failure of both the piping and the welded attachments to the supports. This can result in loss of the indication of core flow, which if it occurs at enough locations, may require plant shutdown. Additionally, in some known plants, licensing changes implement an uprate of the maximum power generated by the plant. The associated increased reactor flow may produce higher excitation frequencies that introduce new resonant vibration response and fatigue failure in the sensing line pipe or its attachments. The following devices are designed to provide an additional sensing line support at a new location such that the natural frequencies of the sensing line are altered to avoid resonance with system excitations.
U.S. Pat. No. 4,675,149 describes a ring and beam clamp supports to provide attachment of the sensing line to the jet pump diffuser. The clamp surrounds the jet pump diffuser to hold the beam assemblies clamped to the sensing line against a spacer fitted against the diffuser wall. Installation requires remote precise measurement of the sensing line position, field trim of the beam spacer to fit, and remote assembly of the beam to the clamp and the jet pump. Also, to allow access for installation, requires disassembly of the jet pumps, which is an added undesirable expense and extension of the reactor outage time.
Another method of attaching or re-attaching the sensing line to the jet pump diffuser uses two bolts to attach a local pipe clamping assembly through holes machined in the diffuser. This clamp design still requires jet pump disassembly for installation access.
U.S. Pat. No. 5,752,807 describes a compact support attachment, utilizing a T-bolt type of blind fastener to clamp the sensing line to the jet pump. This design does not require jet pump disassembly, but does require additional time for machining of a T-slot opening in the diffuser for attachment. U.S. Pat. No. 6,233,301 describes an electrical discharge machining apparatus for machining the T-slot openings in the diffuser.
In one aspect, a clamp assembly for a jet pump sensing line in a nuclear reactor is provided. The clamp assembly includes a clamp body having a bolt cavity, a clamp member coupled to the clamp body, and a clamp bolt sized to be received in the clamp body bolt cavity. The clamp member is configured to extend partially around the jet pump. The clamp assembly further includes a slide member having a bolt receiver and a sensing line receiver, a swing arm having a sensing line recess, and a swing arm bolt sized to be received in the slide member bolt receiver. The slide member is coupled to the clamp body. The swing arm sensing line recess and the slide member sensing line receiver are sized to receive and secure the sensing line, and the clamp member and the clamp body are sized to secure the clamp assembly to the jet pump.
In another aspect, a jet pump assembly is provided. The jet pump assembly includes an inlet mixer, a diffuser coupled to the inlet mixer, a sensing line positioned adjacent said diffuser by a support block; and a clamp assembly. The clamp assembly includes a clamp body having a bolt cavity, a clamp member coupled to the clamp body, and a clamp bolt sized to be received in the clamp body bolt cavity. The clamp member is configured to extend partially around the jet pump. The clamp assembly further includes a slide member having a bolt receiver and a sensing line receiver, a swing arm having a sensing line recess, and a swing arm bolt sized to be received in the slide member bolt receiver. The slide member is coupled to the clamp body. The swing arm sensing line recess and the slide member sensing line receiver are sized to receive and secure the sensing line, and the clamp member and the clamp body are sized to secure the clamp assembly to the jet pump.
In another aspect, a method of clamping a jet pump sensing line to a jet pump within a nuclear reactor pressure vessel, using a clamp assembly is provided. The sensing line includes a support block, and the clamp assembly includes a clamp body, an extended clamp strut, a clamp member, a clamp bolt, a slide member, a swing arm and a swing arm bolt. The method includes positioning the clamp assembly to receive the sensing line in the slide member, positioning the clamp member around the jet pump, positioning the clamp strut adjacent the support block, rotating the clamp bolt to slide the clamp body into contact with the jet pump, to clamp the jet pump between the clamp member and the clamp body, and rotating the swing arm into contact with the sensing line so that the sensing line is secured between the swing arm and the slide member.